Reapers: harvester
Geth: Colossus (and colossi would be the proper plural form)
Cerberus: the gunship sounds like a good idea
Collectors: unless it's also the harvester, I'm at a loss. We already have praetorians, and we can't very well have a wandering proto-reaper popping in.
And we don't need any special animation for them to appear. We already have atlas's magically popping out of thin air, if you're standing in view of a spawn point (whereas in SP, they squeeze through a door, or drop from the sky). I see no reason why these super-bosses can't spawn in like any other enemy.
Though the gunship on Reactor would be a problem. And the only way it would be a threat on Glacier is if you're out in the open on the loading dock. On second thought, maybe the Cerberus super-boss needs a little more thought.
Edit:
Lord of Nightmares posted...Mayb the threat differs depending on which map you're playing? For instance, I can see it being a Thresher Maw on Giant a Colossus on Hydra and possibly an Oculus on Glacier or more confined areas.
Map specific. That would work. And what if whatever it was also attacked enemies? All enemies spawn infinitely on objective waves anyway, so it certainly wouldn't take anything away.
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Gamertag: Talon Omega